The desirability of creating lithography systems which operate with incident light wavelengths below 193 nm to achieve imaging structures or patterns with resolution below 130 nm has been established. In fact, there has been a need for lithography systems using soft X-ray, or so called extreme ultraviolet (EUV), wavelength incident light, such as .lambda.=11 nm or .lambda.=13 nm light, to obtain image resolution in the below 100 nm range. The resolution of a lithography system is described by the following equation: EQU RES=k.sub.1.lambda./NA
where k.sub.1 is a specific parameter of the lithography process, .lambda. is the wavelength of the incident light and NA is the numerical aperture of the system on the image side. This equation can be used to establish design criteria for soft X-ray lithography systems. Thus, for a numerical aperture of 0.10, the imaging of 100 nm structures with 13 nm radiation requires a process with k.sub.1 =0.77. Alternatively, with k.sub.1 =0.64, which occurs with 11 nm radiation, imaging of 70 nm structures is possible with a numerical aperture of 0.10.
For lithography imaging systems in the soft X-ray region, multilayer coated reflective systems, such as Distributed Bragg Reflectors (DBRs) and mirrors, are used as optical components. These multilayer systems employ alternating layers of different indices of refraction. The particular multilayer reflective system depends on the wavelength of incident light used. For example, with incident light of .lambda.=11 nm alternating layers of Mo/Be are preferred and with incident light of .lambda.=13 nm Mo/Si layers are used. In the case of projection objectives used in soft X-ray or EUV microlithography, the reflectivity of these multilayer systems is approximately 70%. It is, therefore, desirable to use as few optical components in the projection objective as possible to ensure sufficient light intensity at the image. With high intensity light, four-mirror projection objective systems were found to be useful in correcting imaging errors at NA=0.10.
Other requirements for soft X-ray projection objectives, in addition to using a limited number of multilayer reflectors, relate to obscuration, image field curvature, distortion, telecentricity on both the image and the object side, free working distance, and aperture stop positioning and accessibility. Obscurations, for example the central obscuration apparent in Schwarzschild systems, create intolerable degradation in image quality. If an obscuration-free light path is required, then, in the case of centered systems, an off-axis image field must be used. In order to provide image formats of 26.times.34 mm.sup.2 or 26.times.52 mm.sup.2, it is advantageous to design the projection objective system as an annular field scanner. In such systems, the useful secant length of the scanning slit should be at least 26 mm, and the annular width should lie in the range of 0.5 mm to 2 mm in order to make uniform illumination and illumination-control and dose-control possible.
Regarding distortion, a distinction between static and dynamic or scan distortion is made. Scan distortion is the effective distortion which is obtained by integration of the static distortion over the scanning path. The limits for magnification-corrected, static distortion follow essentially from the specifications for contrast and CD variation.
Image-side telecentricity is also desired in soft X-ray lithography systems. Whether telecentricity is possible on the object side depends on the type of projection application used. If the projection system uses a reflection mask, then a telecentric optical path on the object side is not possible. If transmission masks, for example stencil masks, are used then a telecentric optical path on the object side can be realized.
In order to enable clean limitations of the beam, the aperture stop should be physically accessible. The image-side telecentricity requirement, referred to above, means that the entrance pupil of the last mirror would lie in or near its focal point. In order to obtain a compact design and maintain an accessible aperture stop, it is recommended that a beam-limiting element be placed before the last mirror. This, preferably, results in the place of the beam-limiting element at the third mirror.
Four-mirror projection or reduction objectives have become known from the following publications:
U.S. Pat. No. 5,315,629 PA1 EP 480,617 PA1 U.S. Pat. No. 5,063,586 PA1 EP 422,853 PA1 Donald W. Sweeney, Russ Hudyma, Henry N. Chapman, David Shafer, EUV optical Design for a 100 mm CD Imaging System, 23rd International Symposium of microlithography, SPIE, Santa Clara, Calif., Feb. 22-27, 1998, SPIE Vol. 3331, p. 2ff.
In U.S. Pat. No. 5,315,629, a four-mirror projection objective with NA=0.1, 4.times., 31.25.times.0.5 mm.sup.2 is claimed. The mirror sequence is concave, convex, concave, concave. From EP 480,617, two NA=0.1, 5.times., 25.times.2 mm.sup.2 systems have become known. The mirror sequence is concave, convex, arbitrary/convex, concave.
The systems according to U.S. Pat. No. 5,063,586 and EP 422,853 have a rectangular image field, for example, at least 5.times.5 mm.sup.2, and use a mirror sequence of convex, concave, convex, concave. These generally decentered systems exhibit very high distortion values. Therefore, the objectives can only be used in steppers with distortion correction on the reticle. However, the high level of distortion makes such objectives impractical in the structural resolution regions discussed here (.ltoreq.130 nm).
From U.S. Pat. No. 5,153,898, overall arbitrary three to five-multilayer mirror systems have become known. However, the disclosed embodiments all describe three-mirror systems with a rectangular field and small numerical aperture (NA&lt;0.04). Therefore, the systems described therein can only image structures above 0.25 .mu.m in length.
Furthermore, reference is made to T. Jewell: "Optical system design issues in development of projection camera for EUV lithography", Proc. SPIE 2437 (1995) and the citations given there, the entire disclosure of which is incorporated by reference.
In the known systems according to EP 480,617 as well as U.S. Pat. No. 5,315,629 and according to Sweeney, cited above, it was found to be disadvantageous that the outside-axially used part of the primary mirror conflicts with the wafer-side sensor structures of a projection exposure installation when not very large free mechanical working distances greater than 100 mm are realized. By using mirror segments which are placed "near the image field", these conflicts occur only at significantly lower distances (.apprxeq.10 mm).
Thus, it is desired from the prior art to provide a projection objective arrangement which is suitable for lithography with short wavelengths (at least below 193 nm and preferably below 100 nm) which does not have the disadvantages of the prior art mentioned above, uses as few optical elements as possible and, yet, has a sufficiently large aperture and fulfills the telecentricity requirements as well as all other requirements for a projection system operating with incident light of wavelengths below 193 nm.